This invention relates to electrolytic cells, particularly to water electrolytic cells for the production of hydrogen and oxygen and more particularly to methods and apparatus for separating hydrogen and oxygen gaseous products entrained in the aqueous electrolyte solution.
Electrosynthesis is a method for the production of chemical reaction(s) that is electrically driven by passage of an electric current, typically a direct current (DC), in an electrochemical cell through an electrolyte between an anode electrode and a cathode electrode from an external power source. The rate of production is proportional to the current flow in the absence of parasitic reactions. For example, in a liquid alkaline water electrolysis cell, the DC current is passed between the two electrodes in an aqueous electrolyte to split water, the reactant, into component product gases, namely, hydrogen and oxygen where the product gases evolve at the surfaces of the respective electrodes.
Water electrolysers have typically relied on pressure control systems to control the pressure between the two halves of an electrolysis cell to insure that the two gases, namely, oxygen and hydrogen produced in the electrolytic reaction are kept separate and do not mix.
In the conventional mono-polar cell design in wide commercial use today, one cell or one array of (parallel) cells is contained within one functional electrolyser, cell compartment, or individual tank. Each cell is made up of an assembly of electrode pairs in a separate tank where each assembly of electrode pairs connected in parallel acts as a single electrode pair. The connection to the cell is through a limited area contact using an interconnecting bus bar such as that disclosed in Canadian Patent No. 302,737, issued to A. T. Stuart (1930). The current is taken from a portion of a cathode in one cell to the anode of an adjacent cell using point-to-point electrical connections using the above-mentioned bus bar assembly between the cell compartments. The current is usually taken off one electrode at several points and the connection made to the next electrode at several points by means of bolting, welding or similar types of connections and each connection must be able to pass significant current densities.
Most filter press type electrolysers insulate the anodic and cathodic parts of the cell using a variety of materials that may include metals, plastics, rubbers, ceramics and various fibre based structures. In many cases, O-ring grooves are machined into frames or frames are moulded to allow O-rings to be inserted. Typically, at least two different materials from the assembly are necessary to enclose the electrodes in the cell and create channels for electrolyte circulation, reactant feed and product removal.
WO98/29912, published Jul. 9, 1998, in the name of The Electrolyser Corporation Ltd. and Stuart Energy Systems Inc., describes such a mono-polar cell electrolyser system configured in either a series flow of current, in a single stack electrolyser (SSE) or in a parallel flow of current in a multiple stack electrolyser (MSE). Aforesaid WO98/29912 provides details of the components and assembly designs for both SSE and MSE electrolysers.
As used herein, the term xe2x80x9ccellxe2x80x9d or xe2x80x9celectrochemical cellxe2x80x9d refers to a structure comprising at least one pair of electrodes including an anode and a cathode with each being suitably supported within a cell stack configuration. The latter further comprises a series of components such as circulation frames and gaskets through which aqueous electrolyte is circulated and product disengaged. The cell further includes a separator assembly having appropriate means for sealing and mechanically supporting the separator within the enclosure and an end wall used to separate adjacent cells blocks. Multiple cells may be connected either in series or in parallel to form cell stacks and there is no limit on how many cells may be used to form a stack. A cell block is a unit that comprises one or more cell stacks and multiple cell blocks are connected together by an external bus bar. Aforesaid PCT application WO98 29912 describes functional electrolysers comprising one or more cells that are connected together either in parallel, in series, or a combination thereof.
Depending on the configuration of such a cell stack electrochemical system, each includes an end box at each end of each stack in the simplest series configuration or a collection of end boxes attached at the end of each cell block. Alternative embodiments of an electrolyser includes end boxes adapted to be coupled to a horizontal header box when both a parallel and series combination of cells are assembled.
In the operation of the cell stack during electrolysis of the electrolyte, the anode serves to generate oxygen gas whereas the cathode serves to generate hydrogen gas. The two gases are kept separate and distinct by a low permeable membrane separator. The flow of gases and electrolytes are conducted via circulation frames and gasket assemblies which also act to seal one cell component to a second and to contain the electrolyte in a cell stack configuration in analogy to a tank.
The rigid end boxes can serve several functions which include providing a return channel for electrolyte flowing out from the top of the cell in addition to serving as a gas/liquid separation device. The end box may also provide a location for components used for controlling the electrolyte level, such as, liquid level sensors and temperature, i.e. for example heaters, coolers or heat exchangers. In addition, with appropriate sensors in the end boxes individual cell stack electrolyte and gas purity may be monitored. Also, while most of the electrolyte is recirculated through the electrolyser, an electrolyte stream may be taken from each end box to provide external level control, electrolyte density, temperature, cell pressure and gas purity control and monitoring. This stream is returned to either the same end box or mixed with other similar streams and returned to the end boxes. Alternatively, probes may be inserted into the end boxes to control these parameters. An end box may also have a conduit to provide the two phase mixture to the existing liquid in the end box to improve gas liquid separation. End boxes of like type containing the same type of gas can be connected via a header such that they share a common electrolyte level.
Thus, an MSE or SSE electrolyser that produces electrolytic gases from a liquid electrolyte requires the separation of the gas and liquid phases and it also requires circulation of the liquid. If the gas and liquid are intimately mixed when the gas bubbles are small, a foam results which generally results in poor gas/liquid separation. The recirculation of gas from the end box/header into the electrode/separator gap spacing effectively increases the electrolyte resistivity and lowers the operating cell efficiency. This leads to the requirement to build larger geometry conduits in the form of end boxes with a subsequent economic penalty. However, in the operation of a water electrolytic cell that produces hydrogen and oxygen gases, it is most important that there be no mixing or intermingling of the hydrogen- and oxygen-bearing electrolyte solutions. Thus, removal of as much as possible of the entrained gases from the aqueous solution is a prime objective. Further, there is a desirable trend in cell design to minimize the sizes of cells, stacks and associated components, such as end boxes, headers and the like. Therefore, achievement of the gas/liquid separation goals as aforesaid must be satisfied in the context of reduced cell volumes. The present invention addresses the combination of these two needs.
It is an object of the present invention to provide an improved method of and apparatus for hydrogen and oxygen gas separation from aqueous electrolyte solutions in an electrolyzer.
Accordingly, in one aspect the invention provides an improved method of separating hydrogen gas entrained with a first aqueous electrolyte solution and oxygen gas entrained with a second aqueous electrolyte solution of a water electrolyser, said method comprising
producing a first two-phase flow discharge of said hydrogen gas in said first solution;
producing a second two-phase flow discharge of said oxygen gas in said second solution;
feeding said first discharge to a first separation chamber having a portion defining a hydrogen chamber to effect separation of said hydrogen gas from said first discharge;
feeding said second discharge to a second separation chamber having a portion defining an oxygen chamber to effect separation of said oxygen gas from said second discharge;
collecting said hydrogen gas from said hydrogen chamber;
collecting said oxygen gas from said oxygen chamber;
collecting said first discharge;
collecting said second discharge;
the improvement wherein at least one of said first discharge is discharged into said hydrogen chamber and said second discharge is discharged into said oxygen chamber.
By the term xe2x80x9chydrogen chamberxe2x80x9d as used in this specification and claims is meant the essentially hydrogen-containing space of the first separation chamber above the level of liquid in this chamber.
By the term xe2x80x9coxygen chamberxe2x80x9d as used in this specification and claims is meant the essentially oxygen-containing space of the second separation chamber above the level of the liquid in the second chamber.
Thus, by discharging the two-phase flows into the respective gas chambers containing essentially only a hydrogen or oxygen-containing phase above the level of any liquid that may be present in the discharge chambers, physical parameters, such as gravity, centrifugal forces, enhanced residence time within the gas chamber, and the like, provide for more efficient gas/liquid separation. Directing the flow above any liquid level, preferably at least horizontally and, more preferably, vertically as an upwardly projecting fountain or like spray provides better separation than when the two-phase flow discharge is merely poured or merged as a stream into liquid either resident or possibly transient in the chamber or foam.
A most preferred two-phase separator system employs use of a vortex or hydro cyclone principle.
The practice of the invention as hereindefined is of particular value when the chamber is constituted as an end box, wherein the gas/liquid mixture is released near the top of the end box and horizontal head, above the liquid level. Preferably, the mixture is discharged within the end box through a suitably disposed essentially vertical xe2x80x9cchimneyxe2x80x9d as a fountain or like spray above the much reduced gas entrained gas/liquid two phase level established in the end box or header by electrolyte circulation with the cell stack. The addition of an internal fixation within the end box in the form of a xe2x80x9cchimneyxe2x80x9d has the advantages of:
(i) enhancing the separation of gas and liquid and improving the operating efficiency of the electrolyser by minimizing the internal resistance generated by circulation of entrapped gas in the form of bubbles;
(ii) improving the circulation of liquid flow through the end box and into the cell block to allow higher internal flow rates; and
(iii) providing a mechanism to minimize the footprint of an MSE or SSE electrolyser by minimizing the cross sectional area of the end box.
In a further aspect the invention provides an improved water electrolyser for producing hydrogen and oxygen gases comprising
means for producing a first two-phase flow discharge of said hydrogen gas in a first aqueous electrolyte solution;
means for producing a second two-phase flow discharge of said oxygen gas in a second aqueous electrolyte solution;
a first separation chamber having a portion defining a hydrogen chamber;
a second separation chamber having a portion defining an oxygen chamber;
means for feeding said first discharge into said first separation chamber to effect separation of said hydrogen gas from said first discharge;
means for feeding said second discharge into said second separation chamber to effect separation of said oxygen gas from said second discharge;
means for collecting said hydrogen gas from said hydrogen chamber;
means for collecting said oxygen gas from said oxygen chamber;
means for collecting said first discharge from said first separation chamber;
means for collecting said second discharge from said second separation chamber;
the improvement comprising at least one of (a) means for feeding said first discharge into said hydrogen chamber; and (b) means for feeding said second discharge into said oxygen chamber.